1. Field of the Invention
This invention generally relates to alternating current (AC) power measurement and, more particularly, to a system and method for measuring AC power with a contactless voltage waveform shape sensor.
2. Description of the Related Art
The electric energy used in a residence is commonly measured with electric meters that have full access to the electric power conductors feeding that residence. However, the electric meter is typically the property of the local electric utility and the measurements made are generally not available to the residence by automated means. In order for the resident to obtain electric consumption data in near real-time, an additional electric meter is often installed behind the utility's electric meter (at the resident's side). The installation of an additional electric meter generally requires access to the same electric conductors as the utility owned meter. The connections consist of two types: 1) a current sensor which is often made with a ferrite core transformer placed around each conductor and 2) a physical electrical connection to measure the voltage on each conductor.
Placement of the current sensors around the insulated conductors requires no modification to wiring. However, tapping the electrical conductors to make voltage measurements often requires physically accessing dangerously high voltage conductors. For safety reasons, the voltage measurement connections often require the installation of permanent attachment points which increases the cost of installation and the expertise required (often a licensed electrician).
FIG. 14 is a drawing depicting a multi-family metering system (prior art). In a multi-family installation, the number of conventional meters and electrical connections required scales with the number of residences metered. At each 240V split phase electric meter, the following is required:                a connection to neutral, L1, and L2        current loops placed around L1 and L2        
Thus, for 50 apartments, 150 connections and 100 current loops would be required. However, since the meters are all likely on one supply bus, L1, L2, and neutral connection (3 tap points) are needed at the feed bus and then the voltages are delivered to the meters (via an external voltage distribution wire network).
Conventionally, a voltmeter requires a contact connection, and it's bundled with the current sensor and put in the panel; thus requiring installation by an electrician. The power is computed from the current and voltage measurements. The power measurements are sent to a collector. The collector can show the measurements on a server or it can send them to a display for the user to view. Some new components described in the literature are non-contact voltage meters using inexpensive electronics. One example is a magnetometer plate for measuring current outside of the panel, and a calibration load used for scaling of the amperage detected by the magnetometer. A magnetometer is a device that measures the strength or direction of a magnetic field. In this case, the flow of current in the conductor induces a magnetic field which is picked up by the magnetometer.
Non-contact single node monitors, such as the ones advertised by Owl® monitor and the PowerSave EnviR, are easy to install because they do not need to be wired in. The drawback is that they need to be battery powered and they suffer inaccuracies because of the poor voltage estimates. The power calculation they use is: Irms×Vrms. Current transformers provide waveform, phase, and magnitude measurements, and a nominal Vrms is estimated. Distortions result as a result of using the nominal Vrms, and from the failure to account for harmonic power factor, displacement power factor, and harmonic voltage.
FIG. 12 is a block diagram depicting a first power measurement system (prior art). Recent papers describe non-contact meters for single residences consisting of separate volt and current nodes. One paper (Patel et al., “The Design and Evaluation of an End-User-Deployable, Whole House, Contactless Power Consumption Sensor”, CHI 2010: Proceedings of ACM Conference on Human Factors in Computing Systems, April 2010) describes a current shape sensor or magnetometer that is attached to the outside of the electric panel and calibrated. Placement of the sensor on the electric panel is crucial and is guided by 2 LED lights on the sensor. A calibrating unit cycles through a series of known loads (10 W, 100 W, and 200 W) that are pulsed at 1 Hz. This system suffers from the fact that it doesn't actively measure voltage, and instead relies on the one calibration (which has an assumed power factor of 1). This approach improves upon the commercial products that use current sensors as it does not require physical access to the conductors, but does not account for changes in power factor or any changes in voltage. One difference between Patel's system and existing commercial meters is the use of the calibrated magnetometer instead of a current transformer, either of which can be used for current sensing.
In summary, Patel discloses a single node system that performs a power calculation of: Irms×Vrms. The sensor is a magnetometer to determine current that must be calibrated, and a nominal Vrms is estimated. Distortions in power measurements occur as a result of using the nominal Vrms, and the failure to account for harmonic power factor, displacement power factor, and harmonic voltage.
A single node system is also proposed by Nora (Maciej A. Noras, “Solid state electric field sensor”, Proc. ESA Annual Meeting on Electrostatics 2011, and Maciej A. Noras. “Electric field sensor based on a varactor diode/MIS/MOS structure” IEEE, Proceeding from Industry Applications Society Annual Meeting, 2010). Noras' proposed single node monitor calculates power as: sum (I×V). The sensors are current transformers and electrostatic voltage meters. An electrostatic voltage meter determines the voltage of a conductor by measuring the electrostatic field generated by that voltage. The value of Vrms is estimated. While the distortions in measurement are limited to the voltage estimate, the electrostatic voltage meter may consume too much power to be packaged as a self-powered (e.g., battery operated) node. This problem can only be ameliorated by measuring voltage sporadically.
FIG. 13 is a block diagram depicting a second power measurement system (prior art). Another paper (Schmid et al., “Meter Any Wire, Anywhere by Virtualizing the Voltage Channel”, BuildSys, Nov. 10, 2010) describes synchronizing voltage phase and voltage values from the voltage-measuring node to the current node. Schmid describes virtualizing the voltage by implementing separate nodes (contact and non-contact). This allows a homeowner to install it by putting the non-contact node in the panel, and then plugging in the Vnode into any nearby wall socket. This synchronization requirement is similar to the synchrophasors on high voltage lines that send voltage and current samples with GPS synchronized timestamps. This system requires that the current node do all the computing, and it requires the node clocks to remain synchronized. While this approach improves significantly over Patel, the synchronization requires 2-way radios and fairly complex phase locked loops, which impact reliability, complexity, and cost.
It would be advantageous if a more accurate means of measuring power usage existed that used a contactless AC voltage sensor.